1. Field of the invention
The present invention relates to an engine tappet superior in abrasion resistance and a method for manufacturing the same. More particularly, the present invention relates to a sintered layer coated on the surface of an engine tappet, which is superior in abrasion resistance and hardness but shows low fusibility to a cam. The sintered layer is prepared from a binder and a compound selected from the group consisting of carbide, boride and nitride or from a high melting point material with superior hardness but low fusibility to a cam. The tappet provided with such a sintered layer can be used in an over-head cam (OHC) type large-sized diesel engine.
2. Description of the invention
Referring to FIG. 1, there is shown a valve train system to open and close a cylinder valve 1. The valve train system comprises a rocker arm 3, a push rod 4 and a tappet 5. The valve 1 is provided with a valve spring 2 to confer restoring force on the valve 1.
Linked to the bottom of the push rod 4, the tappet 5 executes a sliding motion along the outer circumference of a cam. The tappet 4 moves up and down according to the fluctuation of the outer circumference of the cam 6, subjecting the push rod 4 to reciprocating motion.
In recent years, much effort has been made to develop valve train systems of long life span as the use condition for such valve train systems has been more serious in response to an increasing need for maintenance-free engines of high power and high fuel efficiency.
Any failure in such a valve train system causes incomplete combustion, leading to an increase in exhaust gas and smoke. Also, it may cause the engine to make noise and tremble excessively, deteriorating the performance of the whole automobile system.
Since the tappet 5, playing an important role in the valve train system, allows a sliding friction in the high speed contact with the cam 6, the failures in the tappet 4 come mostly from scuffing due to sliding wear and from pitting, a breakaway phenomenon due to surface fatigue.
In addition, since the scuffing and the pitting on the surface of the tappet 5 extremely aggravate the abrasion, the valve is caused to be untimely opened and closed and is problematic in air-tightening. Accordingly, the fuel is incompletely burnt, which results in the problems of exhaust gas and noise.
If an error exceeding a permissible limit exists in the tappet 5 or excessive abrasion is given to the tappet 5 upon the tappet's slide motion along the outer circumference of the cam 6, the tappet cannot be in accurate perpendicular contact with the cam, leading to a cam walking in which the load works in a parallel direction to a cam shaft
In most cases, the tappet 5 and the cam 6 are made of steel and cast iron. These materials can be endowed with abrasion resistance and fatigue resistance by improving their surface hardness and leaving some compression stress in their surfaces through various surface hardening techniques including high frequency induction hardening, carburizing and nitriding.
Chilled cast iron is characterized by the deposition of primary cementite (Fe3C) which is accomplished by rapidly chilling upon casting. In terms of metallography, chilled cast iron is comprised of a pearlite matrix, which is changed from the austenite by Al transformation, and a needle type cementite structure.
Quench chilled cast iron, obtained by quenching and tempering the chilled cast iron, is comprised of a martensite or tempered martensite matrix, a cementite structure, and a temper carbon structure which is decomposed from the cementite structure.
The reason why these substances are dominantly used for the tappet is that the cementite structure is high in hardness and superior in abrasion resistance and the matrix structures prevent plastic flow from occurring, thereby making the tappet highly resistant to scuffing and pitting.
They, however, are problematic in several aspects on account of their being cast. For example, quench chilled cast iron is obtained following such complicated processes as melting, chilling, annealing, quenching and tempering. In addition, if chemical components, chilling speed and thermal treatment conditions are not constant, the cementite and the carbon are apt to be fluctuated in amount and the hardness of the matrix is also fluctuated due to friction heat and high face pressure, so that the resulting tappet becomes poor in abrasion resistance and also in scuffing resistance.
A tappet of high hardness may be prepared from the carbon steel or alloy steel whose surface becomes martensitic by carburization. This material, however, is found to be of lower abrasion resistance than chilled cast iron because martensite is inferior to cementite in thermal stability and also in hardness.
If cast iron materials are applied for the high efficiency engines in which the contact face pressure between the cam and the tappet continues to increase, the materials are required to undergo highly technical management in production, quality and process as well as are highly resistant to heat. However, the thermal resistance of the cementite cannot allow the materials to endure the high face pressure required by the engines as strong as or stronger than 300 horsepower.
To solve these and related problems several methods have been invented. For example, the tappet 5 is mounted in such a way that it is in contact with the cam 6 at oblique angles, to solve so-called cam walking. This method, however, exerts a negative effect of reducing the contact face between the cam 6 and the tappet 5 and aggravates abrasion. To overcome this, a technique was developed in which the surface of the tappet is subjected to crowning so that a combination of a point contact and a line contact is given between the tappet and the cam, thereby avoiding an extreme edge contact. The technique is still problematic in that, if the crown is small in radius, a cocking load is exerted on the surface of the tappet and jamming occurs between the cam and the tappet, causing an uneven surface or reverse flanks.
Calvin C. Connel suggests in U.S. Pat. No. 4,739,674 that the surface of tappet on which the sliding motion is done be a curved surface with a radius of at least about 1,500 m. However, it is difficult for the cam to have a theoretical curve line. In practice, the cam is usually crooked and thus, it is very difficult to expect that the durability of the tappet can be improved only by the shape of the curved surface. In addition, when the crowning radius is too large, a track of a concentric circle occurs on the account of the difference in curvature between the cam and the tappet, severely abrading the cam. Therefore, limitation is given to the solution using such a structural change.
Other solutions to the above problems are suggested in Japanese Pat. Laid-Open Pub. Nos. Sho. 62-182407 and Sho. 62-185806 which disclose a rocker arm and a tappet whose surfaces are fused with a sintered layer made of a nickel powder and a high temperature hard metal powder comprising a carbide, a nitride or a boride.
These above patents, however, introduce only the concept of fusing such a sintered layer but do not suggest the details for manufacturing techniques or methods. Further, the tappet and the rocker arm are economically unfavorable because their production cost including material cost is high.